There is considerable activity in the art of fractional distillation to improve the operation of a column so that products with desired purity specifications can be produced for minimum operating costs. Optimizing the operation of a distillation column is complicated, however, because of the column's numerous degrees of freedom, which are characterized as independent input variables, some of which, such as feed temperature and reboiler heat flow, are controllable, while others such as ambient temperature and feed composition are uncontrollable.
High purity distillation has long been recognized as presenting one of the most difficult control problems in the petroleum and chemical processing industries. This is because high purity distillation columns are extremely nonlinear and exhibit extreme nonlinear coupling. As an example of the nonlinear nature of high purity distillation, a 10% increase in boilup rate will result in only a moderate increase in the purity of the bottoms product, while a 10% decrease would cause a drastic decrease in the purity of the bottoms product. In addition, a 10% increase in boilup by itself will cause a drastic reduction in the purity of the overhead product.
High purity distillation, however, is an industrially relevant process because it is used to produce high purity feed stocks for processes that must have such feed stocks in order to operate properly and economically. For example, ethylene, propylene and styrene monomers of nearly 100 per cent purity are required for their respective polymerization processes in order to produce polymers with the desired characteristics. Also, for the production of industrial grade acetic acid, levels of less than 200 ppm propanoic acid impurity must be maintained. In addition, chemical intermediate xylene products are typically produced as high purity products. Ethylene oxide and propylene oxide are separated industrially to produce products, each with about 200 ppm impurities.
One of the more important independent variables of a distillation column is the reboiler heat input. This is because distillation is a thermal process, and reboiler heat input may be manipulated to compensate for disturbances in other uncontrollable variables, such as changes in feed composition and feed flow rate. Reflux flow rate is another important independent variable affecting separation which may also be manipulated to compensate for disturbances in the uncontrollable variables. A control system having capability for simultaneously manipulating two variables, such as heat input and reflux flow rate, so as to maintain dual product specifications would be highly desirable.
Industry mainly relies on the classical proportional-integral-derivative (PID) controller or else a linear model based controller for multivariable control applications. Model based controllers, however, provide improved performance over PID controllers in many control applications because it is feasible to automatically update the model to match changing process operating conditions. This permits maintaining near optimum tuning over wide ranges of a process variable. Further, nonlinear models can account for both the nonlinearities of the column and the interactions between manipulated variables of the column to improve control performance compared to systems employing linear models.
Although the theoretical development of steady state, nonlinear process model techniques is considerably advanced, most industrial processes, which employ models, are still based on linear models (i.e. feedforward, cascade, dynamic matrix, and internal model techniques) which may not be characteristic of the process being controlled, and which require a substantial process upset for confidently updating the model to match new process operating conditions.
Accordingly, it is an object of this invention to improve control of a distillation column by using a nonlinear process model for distillation control decisions.
It is another object of this invention to provide a nonlinear model which contains the major nonlinear characteristics of a high purity distillation column.
It is another object of this invention to provide a distillation control scheme which simultaneously manipulates dual distillation variables so as to maintain dual distillation product specifications.
It is another object of this invention to provide a distillation control model which can be updated using steady state values of process parameters.
It is another object of this invention to provide control action which is optimally tuned for both small and large changes in controller output.
It is a still further object of this invention to provide substantial energy and material savings in a petroleum or petrochemical refining process while producing more uniform products.
It is a still further object of this invention to provide a control system for confidently operating a distillation column at composition set points for both overhead and bottoms products essentially matching product sales purity specifications, without failing to achieve the required purity.